The term "integrated optics" refers to a class of devices for guiding and controlling light in thin film layers or in narrow waveguide channels formed in a suitable dielectric material. The integrated optic (I/O) devices can be either of a single type including transducers, filters, modulators, memory elements, and others or of several functional applications combined ("integrated") onto a single device.
Although several materials have been used for I/O device fabrication, one of the most widely used I/O device materials is lithium niobate. Lithium niobate is used primarily because of its optical and electro-optical properties. At room temperature, the atomic structure of lithium niobate belongs to the rhombohedral (trigonal) space group R3c and where the point group is 3 m. This formula for lithium niobate is stable up to the ferroelectric transition around 1200.degree. C., where a transition to the nonpolar point group 3 m occurs. Crystallagraphers can measure parameters for lithium niobate as either a hexagonal or a rhombohedral unit cell. When measured as a rhombohedral unit cell, the atomic structure of lithium niobate exhibits an interaxial angle of approximately 56.degree. and a unit cell axis length of approximately 5.5 angstroms. As a hexagonal unit cell, the atomic structure of lithium niobate has a unit cell axis length (a=b) of approximately 5.5 angstroms and a c axis length of approximately 13.8 angstroms. For purposes of this application the hexagonal unit cell shall be used to define the atomic structure of lithium niobate.
Furthermore, lithium niobate is characterized by having a large pyroelectric (i.e., spontaneous polarization of the crystal as a function of temperature), piezoelectric (i.e., induced polarization as a function of applied stress), photo-elastic (i.e., change in refractive index as a function of applied stress), and electro-optic (i.e., change in refractive index as a function of applied electric field) properties. It is also birefringent (i.e., linearly polarized electromagnetic waves will travel at two different velocities and along two perpendicular principal displacement directions) and exhibits a very strong bulk photovoltaic effect which also can produce a significant photo-refractive effect (i.e., change in refractive index as a function of the change in the optical intensity of the propagating light). See R. S. Weis and T. K. Gaylord: Appl. Phys. A37, P. 191-203 (1985).
With all these optical and electro-optical properties, lithium niobate has found widespread application in laboratory and experimental systems. However, in order to make practical use of lithium niobate as an integrated optic device, numerous material problems still require solutions. One such problem is that lithium niobate devices still require a coupling device between themselves and optical fibers which functions over a wide range of environments (i.e., shock, vibration, and temperature). This inability to develop an environmentally stable coupling device stems from lithium niobate's strong anisotropic thermal expansion properties and from movement between the waveguide and the optical fiber. A strong anisotropic thermal expansion property means that the dimensional changes in the material associated with a temperature change differs in different directions in the crystal. Lithium niobate exhibits a thermal expansion property along the Z-axis (the Z-axis being defined as the axis about which the crystal exhibits three-fold rotation symmetry) in the range between 2.times.10.sup.-6 /.degree.C. to 7.5.times.10.sup.-6 /.degree.C. (Note: the variations being due to the various investigators' use of different materials, measurement techniques and over different temperature ranges), while the thermal expansion in the isotropic X or Y axes are in the range between 14.times.10.sup.-6 /.degree.C. to 17.times.10.sup.-6 /.degree.C. Since in practical military applications where the integrated optic device must survive temperature fluctuations between -40.degree. C. to +80.degree. C., the anisotropic thermal expansion property of both the integrated optic device and the coupling device must be substantially similar to maintain the bond between them. Movement between the optical fiber and the waveguide occurs primarily because of the environment in which the integrated optic device is used along with the anisotropic thermal expansion property of lithium niobate In a gyroscope system, for example, the integrated optical device will have one degree of freedom which will sustain a substantial amount of the shock occurring along that axis while all three axes will undergo vibration. Since the light wavelength in many state of the art military applications is at the 850 nanometer range, the outside maximum allowable movement between the waveguide core with respect to the optical fiber core or vice versa is only 0.5 microns Any movement beyond this narrow tolerance causes totally unacceptable distortion to the output signal.
Despite these obstacles, several attempts at developing an environmentally stable coupling device for 850 nanometer optical fiber have been attempted One such attempt used ion milled grooves in the lithium niobate device which permitted the rigid location of a chemically etched and polished optical fiber. This approach had alignment grooves which were defined by conventional photolithography and which were fabricated using ion milling in the same substrate as the waveguide. The coupling of the optical fiber to the waveguide was performed by inserting and bonding an etched portion of an optical fiber into the ion milled groove. Hence, the light emerging from the etched fiber went directly into the waveguide. See A. C. G. Nutt et al: Optics Letters, Vol. 9, No. 10, P. 463-465 (Oct. 1984). Although a good approach, this attempt had several shortcomings. First, the ion milling of the grooves in the waveguide and the etching of the optical fibers was a slow, expensive, and time consuming process requiring great precision which allowed for potential alignment problems between the waveguide and the optical fiber Second, the etching of the fibers greatly reduces the polarization maintenance properties (i.e., stress zones) of the optical fibers which may exclude the use of birefringent optical fibers. Third, this approach was never actually demonstrated over a wide temperature range or for use with low loss connectors.
Another attempt at providing a temperature stable coupling device was the use of silicon V-grooves. Optical fibers were prepared by epoxying single-mode fibers into V-grooves etched onto a silicon chip. A silicon chip cover was then mated with the silicon chip containing the etched V-grooves so that the optical fiber cores were precisely and periodically spaced along a straight line. After this assembly of the silicon chip, the end faces of the optical fibers were polished, butt coupled to a corresponding lithium niobate waveguide, aligned, and then attached using an optical adhesive. See E. J. Murphy et al: J. of Lightwave Tec., Vol. LT-3, No. 4, P. 795-798 (Aug. 1985). Although this approach allowed multiple optical fiber mountings to the waveguide, it also had problems. First, the silicon V-groove chip required very great accuracy and precision in the etching of the V-grooves in order to allow for the correct alignment of the optical fiber cores to the waveguide core. Second, the silicon V-groove approach could only effectively use single mode optical fiber with a 1300 nanometer wavelength or the 850 nanometer wavelength with reduced performance. This approach has never been demonstrated using either polarization maintaining optical fibers or optical fibers at the 850 nanometer wavelength. Third, the silicon V-groove chip is relatively large in physical size. Finally, the silicon V-grooved chip and the integrated optical device have different thermal expansion properties. Hence, at the silicon-lithium niobate interface, a substantial thermal mismatch exists which can result in thermal instabilities which could destroy the bond between optical fiber and the waveguide.
It should be noted at this point that the coordinate system used to describe the physical tensor properties of lithium niobate is neither hexagonal nor rhombohedral but rather an X-Y-Z Cartesian coordinate system. Lithium niobate exhibits an anisotropic thermal expansion coefficient along an optical or C-axis and an isotropic thermal expansion coefficients along the other two axes. A light beam projected along the optic axis has the same refractive index regardless of the polarization of light and hence light propagating along this axis is said to be "symmetric". Accordingly, the optic axis which is usually assigned to the Z-axis can be easily determined. The sense of the Z-axis is the same as that of the optic axis. (i.e., upon compression the +Z face becomes negatively charged because of the piezoelectric effect.) As stated earlier in this background art section, lithium niobate has an atomic structure which can be described as a conventional hexagonal unit cell. According to the standard definition for a hexagonal unit cell, a hexagonal unit cell has three coplaner axes, called a.sub.H axes, 120.degree. apart from each other and equal in magnitude to each other along with a fourth axis, called a C-axis or in the case of lithium niobate the optic axis, which is at right angles to the three a.sub.H axes. Also, optic axis of lithium niobate lies in three mirror planes of symmetry about which any charge movement on one side of the plane is "mirrored" by movement on the other side. These three mirror planes of symmetry are also perpendicular to the a.sub.H axes. The X-axis is chosen to coincide with any of the a.sub.H axes and, since a right-handed cartesian coordinate system is used, the Y-axis lies in a plane of mirror symmetry. The sense of the Y-axis is determined in a manner similar to that described for the Z-axis. (I.e., upon compression, the +Y face becomes negatively charged because of the piezoelectric effect.) The sense of the X direction, however, cannot be determined in this way because the X-axis is perpendicular to a mirror plane. Any charge movement on one side of the plane is "mirrored" on the opposite side, hence, the X faces do not become charged.
Typically, suppliers of lithium niobate crystals furnish pieces that are commonly in the form of thin slabs. These thin slabs may be designated X-cut, Y-cut, or Z-cut respectively, to the X, Y, or Z axes being normal to the broad face of the slab. A second letter is often added to the slab orientation indicating the direction of the propagation of light through a waveguide within the slab. Thus, an "X-cut, Y-propagation" describes a device having the X-axis normal to the broad face and the Y-axis in the direction in which light propagates within the waveguide.